The present invention relates to compact motors used in vehicles, audio equipment, video equipment, precision equipment and the like. More specifically, it relates to a compact motor with which cogging torque can be reduced without lowering output torque.
In recent years, with the development of vehicles and various types of precision equipment, the demand for compact motors having low cogging while maintaining high torque is increasing.
Initially, coreless motors, which to not have an armature core, were used as a countermeasure against cogging but, as coreless motors produce relatively low torques, there was a problem in that, in order to produce a large torque, it was necessary to increase the overall size of the motor.
Meanwhile, when large torque is required of compact motors, motors are used in which, as shown in FIG. 8, permanent magnets 103, which have been given alternating north-pole and south-pole magnetizations, are arranged within a case (magnetic yoke) 104, and an armature core 101 having salient poles 102 is disposed facing the inner sides of these permanent magnets 103.
While motors having an armature core of this sort can produce a large torque, there was a problem in that, because magnetic permeability intermittently changes when the boundaries of the magnetic poles of the permanent magnets 103 cross the slots between the salient poles 102, a torque variation is generated, having a frequency that is synchronized with the rotation speed, which is known as cogging.
For this reason, methods for reducing cogging by providing grooves in the salient poles of an armature core have been proposed, for example in JP-61-030945-A. Furthermore, in recent years, when large torques are required of high-quality compact motors, rare earth magnets having strong magnetic force have come to be used for the permanent magnets 103. Examples of methods for magnetizing the permanent magnets include the method wherein, as shown in FIG. 9, magnetization is performed from the exterior of the case 104.
In FIG. 9, 111 indicates magnetizing yokes, on which a magnetizing coil 112 is wound; a solid cylindrical internal yoke 114 is interposed within a sheet member 113 principally comprising a powdered rare earth magnet material and rubber, the sheet member 113 having been affixed to the inner circumferential face of a case 104, which is positioned between a pair of the magnetizing yokes 111. The sheet member 113 is magnetized to a substantially sinusoidal magnetization waveform, for example, by way of applying a DC pulsed voltage to the magnetizing coil 112.